packaged-staging.bbclass: fix issue with no SRC_URI

packaged-staging.bbclass: fix issue with no SRC_URI

Currently with srctree, we get errors like the following:
| + srcuri=
| + '[' '' == '' ']'
| + srcuri=OpenEmbedded
| + echo 'Source: '
| ++ find /scratch/oe/oe-build/oe/build/angstrom-2008.1/tmp/work/armv5te-angstrom-linux-gnueabi/autotools-demo-1.0-28-g3a96627-r0/staging-pkg '(' -name '*.la' -o -name '*-config' ')' -type f
| + stage-manager-ipkg-build -o 0 -g 0 /scratch/oe/oe-build/oe/build/angstrom-2008.1/tmp/work/armv5te-angstrom-linux-gnueabi/autotools-demo-1.0-28-g3a96627-r0/staging-pkg /scratch/oe/oe-build/oe/build/angstrom-2008.1/tmp/pstage/angstromglibc
| *** Error: CONTROL/control is missing field Source
| The Source field contain the URL's or filenames of the source code and any patches
| used to build this package.  Either gnu-style tarballs or Debian source packages
| are acceptable.  Relative filenames may be used if they are distributed in the same
| directory as the .ipk file.
|
| ipkg-build: Please fix the above errors and try again.
| ERROR: Function staging_packager failed
NOTE: package autotools-demo-1.0-28-g3a96627-r0: task do_package_stage: Failed
ERROR: TaskFailed event exception, aborting

This patch fixes that issue.

Acked-by: Chris Larson <chris_larson@mentor.com>
Acked-by: Koen Kooi <k-kooi@ti.com>
Acked-by: Khem Raj <raj.khem@gmail.com